In producing ultrapure water required in technical fields such as cleaning of electronic parts in the electronics industry, manufacture of pharmaceutical preparations, etc., pure water produced by primary purification such as distillation or ion exchange is supplied to an ultrapure-water producing device containing a polisher (a well-known device for improving the purity of water, which a resin is packed in) and the like to further remove impurities including particles to thereby improve the purity of the water. The ultrapure water thus produced is fed to a use point through a feed line and is used for cleaning or other purposes. Thereafter, the unused ultrapure water or the spent pure water is circulated to the primary purification step and reused.
Illustratively stated, as shown in FIG. 3, pure water produced in a pure-water producing device 25 and stored in a primary-pure-water tank 26 is sent to an ultrapure-water producing device 28 by means of a pump 27. The ultrapure-water producing device 28 comprises at least a polisher 29 which removes inorganic substances from the pure water and a precision filter 30, e.g., a microfilter or an ultrafiltration membrane, which removes fine particles and organic polymers, and serves to improve the purity of the supplied pure water and filter off the resulting impurities. The ultrapure-water producing device 28 may contain an ultraviolet sterilization lamp for destroying live bacteria. In the case shown in FIG. 3, the ultrapure water produced by the ultrapure-water producing device 28 is fed through a feed line 31 to two use points 32, where a treatment, e.g., cleaning of electronic parts, is conducted. The spent pure water resulting from this treatment or the unused ultrapure water which has not been used in the treatment because of the excess amount fed is collected and circulated to the primary-pure-water tank 26 through a return line 33, and is then resent to the ultrapure-water producing device 28 and reused for cleaning or other purposes.
In this treating system shown in FIG. 3, which is based on the circulation of ultrapure water, the ultrapure water is used effectively and the treatment can be conducted efficiently as described above. However, even when the ultraviolet sterilization lamp is used, the sterilization capability of the lamp is so low that the growth of bacteria is observed even when the circulating pump 27 is operated to maintain the circulatory flow so as to avoid water stagnation. The bacteria deteriorate the quality of the ultrapure water and the treatment with this quality-deteriorated ultrapure water at the use points leads to impaired quality of the resulting treated products and to a decrease in yield or production efficiency. As the number of bacteria thus increases, the number of particles present in the ultrapure water in the system and the TOC (total organic carbon) of the water tend to increase.
Hitherto, the generally employed technique for keeping the number of bacteria which inevitably multiply in the ultrapure-water circulating system at the lowest possible level has been to periodically stop the operation and to add either a germicide, e.g., sodium hypochlorite, hydrogen peroxide and ozone, or hot water to the system, to thereby sterilize and clean the inside of the system.
Although this effective technique is in inhibiting the multiplication of bacteria, this technique is disadvantageous in that it is necessary to stop the operation of the ultrapure-water circulating system for a few hours to one day. Hence, the production of ultrapure water and the manufacturing process employing the ultrapure water must be stopped or slowed down during that period. In particular, in the electronics industry where year-round continuous operation is desired, it is undesirable to employ the above-described technique which leads to a decrease in production efficiency.
Furthermore, the technique of adding a germicide results in secondary problems. For example, it is necessary that after the use of a germicide for the system to be thoroughly post-cleaned with ultrapure water so as to prevent the germicide from remaining in the system, and this post-cleaning causes a problem also in the treatment of the resulting wastewater. In the case of using hot water, there are problems concerning the consumption of heat energy required for heating, the heat resistance of organic and other materials employed in the system piping, etc., which problems remain unsolved.
Further, to improve the efficiency of treatment, e.g., cleaning of parts, an attempt has been to dissolve ozone into the ultrapure water and use this ozone-containing ultrapure water for a treatment such as the cleaning described above.
Ozone has a higher oxidizing power than chlorine, is effective in sterilization, deodorizing, decoloring, etc., and does not pose any problem concerning secondary environmental pollution because it decomposes to oxygen relatively readily after serving its oxidative function. Hence, the range of ozone utilization is expanding recently.
However, the decomposition does not proceed completely, and ozone remains in a slight amount. In addition, the spent ultrapure water also contains oxygen dissolved therein which is the decomposition product. At present, treatment with active carbon is performed in order to completely remove these residual gases before the water is discarded as an effluent. This treatment, however, results in the presence of a large amount of impurities attributable to the active carbon. The amount of impurity is so large that the treated water cannot virtually be returned to the above-described ultrapure-water circulating system and reused. Since residual gases adversely affect the ultrapure-water producing device etc., circulation of the spent pure water which has not been treated to remove gases is not adequate.
In order to eliminate these drawbacks, a technique has been proposed in which the piping is formed of a material unsuited for bacterial growth and a low-pressure ultraviolet ozone decomposer is disposed immediately before the ultrapure-water producing device to conduct ozone decomposition (unexamined published Japanese patent application No. 2144195). However, this technique has the drawback that the complete decomposition of the ozone with a low-pressure ultraviolet ozone decomposer is difficult and ozone remains in the pure water to be fed to the ultrapure-water producing device, with the ultrapure-water producing device tending not to fully exhibit its performance.